There is no plan to reapply the kernel side of these fixes ahead of
18.04.4. Introducing the mdadm side of that in 18.04.4 therefore would
cause a regression in functionality:
ubuntu@ip-172-30-0-208:~$ sudo mdadm --create /dev/md0 --run
--metadata=default --level=0 --raid-devices=2 /dev/xvdb /dev/xvdc
mdadm: /dev/xvdb appears to be part of a raid array:
level=raid0 devices=2 ctime=Wed Dec 11 22:10:59 2019
mdadm: /dev/xvdc appears to be part of a raid array:
level=raid0 devices=2 ctime=Wed Dec 11 22:10:59 2019
mdadm: ADD_NEW_DISK for /dev/xvdb failed: Invalid argument
mdadm: Possibly your kernel doesn't support RAID0 layouts.
mdadm: Either upgrade, or use --layout=dangerous
Of course, at install time, upgrading the kernel isn't an option. So
let's hold this SRU until after the point release.
** Description changed:
Bug 1849682 tracks the temporarily revert of the fix for this issue,
while this bug tracks the re-application of that fix once we have a full
solution.
[Impact]
(cut & paste from https://marc.info/?l=linux-raid&m=157360088014027&w=2)
An unintentional RAID0 layout change was introduced in the v3.14 kernel. This
effectively means there are 2 different layouts Linux will use to write data to
RAID0 arrays in the wild - the “pre-3.14” way and the “3.14 and later” way.
Mixing these layouts by writing to an array while booted on these different
kernel versions can lead to corruption.
Note that this only impacts RAID0 arrays that include devices of
different sizes. If your devices are all the same size, both layouts are
equivalent, and your array is not at risk of corruption due to this
issue.
Unfortunately, the kernel cannot detect which layout was used for writes
to pre-existing arrays, and therefore requires input from the
administrator. This input can be provided via the kernel command line
with the raid0.default_layout=<N> parameter, or by setting the
default_layout module parameter when loading the raid0 module. With a
new enough version of mdadm (>= 4.2, or equivalent distro backports),
you can set the layout version when assembling a stopped array. For
example:
mdadm --stop /dev/md0
mdadm --assemble -U layout-alternate /dev/md0 /dev/sda1 /dev/sda2
See the mdadm manpage for more details. Once set in this manner, the layout
will be recorded in the array and will not need to be explicitly specified in
the future.
(The mdadm part of this SRU is for the above support ^)
[Test Case]
= mdadm =
Confirm that a multi-zone raid0 created w/ older mdadm is able to be started
on a fixed kernel by setting a layout.
1) Ex: w/ old kernel/mdadm:
mdadm --create /dev/md0 --run --metadata=default \
--level=0 --raid-devices=2 /dev/vdb1 /dev/vdc1
2) Reboot onto fixed kernel & update mdadm
3) sudo mdadm --stop /dev/md0 &&
- sudo mdadm --assemble -U layout-alternate \
+ sudo mdadm --assemble -U layout-alternate \
/dev/md0 /dev/vdb1 /dev/vdc1
4) Confirm that the array autostarts on reboot
5) Confirm that w/ new kernel & new mdadm, a user can create and start an
array in a backwards-compatible fashion (i.e. w/o an explicit layout).
6) Verify that 'mdadm --detail /dev/md0' displays the layout
= linux =
Similar to above, but using kernel command line options.
[Regression Risk]
The kernel side of things will break starting pre-existing arrays. That's
intentional.
- Although I've done due-diligence to check for backwards compatibility
- issues, the mdadm side may still present some.
+ The mdadm side will cause a regression in functionality where a user can
+ no longer create multi-zone raid0s on kernels that do not yet have the
+ raid0 layout patches. This is intentional, as such RAID arrays present a
+ corruption risk.
** Tags removed: verification-done verification-done-bionic
verification-done-disco verification-done-eoan
** Tags added: verification-needed verification-needed-bionic
verification-needed-disco verification-needed-eoan
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https://bugs.launchpad.net/bugs/1850540
Title:
multi-zone raid0 corruption
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